I have no idea about approach grafting. Up here where I live the growing season is so piddly that we can't approach graft much of anything usually. We have to, for instance, send our local junipers down to Southern California if we want to have Shimpaku grafted on, even though Shimpaku grows great here on its own.
But, having said that, the MHs have this weird thing going where wrapped wire will sit there, and sit there, and sit there, and sit there - and then suddenly, out of the blue, if you haven't inspected it for a week it will suddenly thicken horribly along one stretch and bury the wire almost overnight, creating an almost permanent scarring. Now IF we could get it to do some sort of trick like that with an approach graft we might be in luck!